In a storage phosphor imaging system, as described in U.S. Pat. No. Re. 31,847, reissued Mar. 12, 1985 to Luckey, a storage phosphor is exposed to an x-ray image of an object to record a latent image in the storage phosphor. The latent x-ray image is read out by stimulating the storage phosphor with a relatively long wavelength simulating radiation, such as red or infrared light. Upon stimulation, the storage phosphor releases emitted radiation of an intermediate wavelength, such as blue light, in proportion to the quantity of x-rays that were received. To produce a signal useful in electronic image processing, the storage phosphor is scanned in a raster pattern by a laser beam deflected by an oscillating or rotating scanning mirror. The emitted radiation from the storage phosphor is collected by a light collector and detected by a photodetector, such as a photomultiplier to produce an electronic image signal. Typically, the storage phosphor is translated in a page scan direction past a laser beam, that is repeatedly deflected in a line scan direction perpendicular to the motion of storage phosphor to form a scanning raster.
In order to optimize a signal-to-noise ratio of the imaging system, it is desirable to collect as much of the emitted light as possible, and to direct it to the photodetector. An easily manufacturable, low-cost, high-efficiency light collector is disclosed in commonly assigned U.S. Pat. No. 4,743,759, issued May 10, 1988, inventor John C. Boutet. Even though the light collected for detection by a photodetector may be collected highly efficiently, noise components are introduced into the detected image signal by the scanning laser beam. The noise fluctuations in the scanning laser beam cause the light emitted from the storage phosphor to also fluctuate, introducing false readings in the detected image signal. A proposal to reduce the effects of laser induced noise described in U.S. Pat. No. 4,410,799 issued Oct. 19, 1983, inventor Okamoto. As disclosed in the latter patent, a radiation image information readout apparatus reads out a stimulable phosphor in which a radiation image is recorded by use of stimulating rays impinging thereon. The gain of the readout system is controlled, according to a fluctuation of the intensity of the stimulating ray source, by use of an additional photodetector, which detects the laser intensity before the stimulable phosphor is scanned. A similar system is disclosed in U.S. Pat. No. 4,150,402, issued Apr. 17, 1979, inventors Tietze et al, in which the effect of laser noise in a scanning laser read system is reduced by sampling the laser beam prior to scanning a document. The sampled laser beam is converted to an electrical signal which is amplified and applied to a gain control device, such that noise present in the output laser beam modulates the signal detected from scanning the document to eliminate laser noise.
Another proposal to reduce laser generated noise in a storage phosphor system is disclosed in U.S. Pat. No. 4,896,222, issued Jan. 23, 1990, inventor Fukai. As disclosed in the latter patent, an object such as a stimulable phosphor sheet, on which image information is recorded, is scanned by a laser beam, and emitted light is photoelectrically detected to produce an image signal representing the image formation. A monitor signal representing the light beam is detected by a photodetector and is divided into a high frequency component and a low frequency component. A corrective signal is produced by multiplying a ratio of the high frequency component to the low frequency component by a constant. This corrective signal is then subtracted from the image signal.
U.S. Pat. No. 4,977,322, issued Dec. 11, 1990, inventors, Agano et al., discloses a technique to minimize image signal noise in a stimulable phosphor system.As disclosed, a detected image signal is fed to a high frequency component decreasing device for decreasing the high frequency component, the level of the image signal is detected,and the high frequency component of the image signal is decreased by the high frequency component decreasing device, in accordance with a decrease in the level of the image signal.
There is thus a problem in the prior art of laser beam scanning systems to correct for signal fluctuations in a scanned image produced by noise in the scanning laser beam.